The present invention relates to a receiving system, comprising a high frequency section followed by a low frequency section, and wherein the high frequency section is realized in analog technique, whereas the low frequency section is realized in digital technique. The high frequency section includes an input preamplifier followed by two quadrature actuated mixers, with associated .pi./2 phase shifter and voltage controlled oscillator, wherein the high frequency signal is directly converted into the base band by multiplication by means of a phase locked control loop, and respective lowpass filters connected to the outputs of the mixers to select out interfering mixed products.
Present-day receivers for radio and television are based on the structure of the superheterodyne receiver whose block circuit diagram is shown in FIG. 1. The received signal is fed to a mixer M via a selective low-noise preamplifier VV whose center frequency is variable, and is converted to a constant intermediate frequency. Thereafter the actual frequency selection is made in the further selective amplifier V', the selected signal is demodulated in a demodulator Dem, and the demodulated signal is amplified in the amplifier V". This receiver concept requires a very constant-frequency mixing oscillator and highly selective bandpass filters, which are expensive to tune, in the amplifier for the frequency selection. For a long time, this process has been adapted to technological advances and has now reached a high degree of development.